Signal voltage frequency converter



Oct. 28, 1952 T. E.JAcoB| ET Al.

SIGNAL VOLTAGE FREQUENCY CONVERTER Filed June 16, 1945 uw L ATTORNEY KQ Nr Patented Oct. 28, 1952 UNITED STATES F'ETENT F i? ECE LQ Gensel, Philadelphia, Pa-. assignors to RadioV Corporation of 'AL-merica, ajcorporation of Dela- Ware.

Application- .I'une 16, 1945,` Serial No. 599,830.

3; Claims. (Cl'. 332-48)- This application relates to signalling means,

andjin particular to improvedmeans` for supplyingY oscillations to signalling circuits-L in each of whichtheroscillations maybe used' asdesired 'with othercurrents or oscillations.

In someV applications such as, for example, twoway-y communication wherein oscillations are used in thev sending channels and oscillations offline frequency are used inthe receiving channels, it is` necessary that theoscillati-ons be of' the saine frequency. To'simpliiy suchr arrangemen-tswe make use of aV single oscillator coupled toboth channelsI so that oscillations supplied to the channels'are of the samefrequency;

When oscillationsv from a common source are supplied to circuits one or both of which carry other currents, there is dangei'i'fl'iatthefsaidl two circuits` will bef'coupled. by the oscillation supply connections, so that the currents' in one or both Yof the circuits will interfere with the currents in the other circuit. An exampleof such an` arrangement: is found in communication systems whereintwo -way communication from each station isI accomplished by' usingateach station a common oscillator which provides oscillations to: invertthe' signals to be sent out and' also` sup.- pli'esoscillationstoreinvert and recoverY the message froxnthe incoming signals.

An! object of ourA invention is an improved one way coupling between a generator and two or more channels wherein oscillations from the generatorareused.

A more speciiic object of our invention is. im'- proved couplings between a common` generator and the Ltransmitting` inverting converter or modulator andamplier; and between the common generator andthekv receivine reinverting converter or, modulator and' amplifier in an invertedspeech` or other signal'A communication sys:- tern.

Byiinverting, the signals secrecy is attained. ".llo invert the signals the, oscillatorfrequency'may be located in the upper margini of the signal frequency spectrum andthe difference frequency selected' so that the signals originally 'of' high frequency are of low frequency andvice versa. The invertedY signals `are reinverted to recover the signals.

Thel specic, example given above is to facili'- tate, description of our inventionandl understand,-

ingI thereof. It will be obviousfto. thoseversed 2. but isolate the circuits each from the others with respect. to currents in the circuits. In the description the'oscillations are referred to as carrier oscillations since that is one vof their puriposes, even though they are in the speech frequency range; Obviously, theyfmay be of superaudible or subaudible frequency and may still serve in one channel as the, carrier which is modulated by the signals and in the other channel' asl the beating oscillations to recover the signals from the incoming carrier.

One of the channels may have as input a radio receiver'outputandl as an output the signal, while the other channel may have as input the out" put, of a microphone and as output. amplied signals for sending; In many cases' the output terminals of onev channel' andv the inputV terminals of the other channel are connected to the sub,- scribers station telephone headsetY and microphone through a hybrid coupling unit and a two wireline; Obviously, it is` necessary to provide meansfor attenuatingthe carrier or oscillator frequency fedA back into the input, side of` the channels or the oscillations4 couple through the hybrid unit'and' reach the subscriberV atY the head'- Str.

An: object of; ourl invention is to provide improved coupling meansas described above including meansarranged for attenuating' the carrier frequency fed backV to theA input of the chan'- nels'or-linesv to prevent the oscillations from feeding back over the lines to the subscriber;

In the example given, the oscillations serve as carrier or invertingY oscillationsA in a speech inverter system and-'the oscillations arefed into a line to be mixedtherein withI thev audio potentials' or signals; An object; of' our invention is to provide arlimproved mixing or combining are rangement; Y Y In describingA ourl invention in.v detail reference willbev madeto the attached drawing, wherein` the, single figure illustrates a common oscillatorA feeding oscillations; to two channels of a speech signalling system. arranged in ac;- cord'ance withv our invention, and inclu-dingjthe features: enumerated vabove and others. f

In the drawings oscillatory energy such as voiceV signals derived for `example from,V a. radio receiver i0. areto be translated over channel #l through an fattelrnuatingY network AT andthe lowA passlter X9 to a converterI 212,'` and` thence through a low pass lter Xl to an output, vdevice, which. may 'be a telephonefunitg A'udio frequency currents from4 a source such,y as, a microphone, for example, are tol be translated over channel #2 thoughfattenuation network A'll'- low pass` filter XH, converterv 26 and` low pass lter X'i2f`to for e}' a' 1nple`a radio transmitter or other. transmitter. n

` The. audiov rsignalsf.passedby filters X9 and vX-II.

are to be mixed with oscillations from the source 20 in the converters 22 and 26 and this is to be done without material coupling between the channels and Without material feedback through the low pass lters X9 and XII to the inputs of the two channels. The source of oscillations 20 which is designated the source of carrier signal may be of audio frequency or above or below audibility. oscillations from 20 are fed to the grids 34 and 36 of a double triode electron tube structure. The excitation circuits include series resistors 49 and 42 which serve as grid excitation reducing inipedances. The grid direct current circuits are completed through one winding of the transformer associated with source 2G. The anodes of the tube systems are connected together and to the positive terminal of a source of direct current potential having its negative terminal grounded. The anodes are also substantially grounded, with respect to the alternating currents in the outputs ofthe cathode followers, by a capacitor 43 of appropriate size. The cathodes 44 and 46 of the electron tube systems are connected to ground by cathode return resistances 48 and 50. These resistances also serve as the output impedances of the two triode systems. The electron tube systems'may be in a single envelope as shown, or may be in separate envelopes.

The cathode impedance or load of one tube system, say for example, the impedance 48, is coupled by a coupling and direct current blocking capacitor 52 to the primary winding of a transformer T9 in series with the low pass filter XII. The other output or load impedance 50 is similarly coup-led by a coupling and blocking capacitor 54 to the primary of transformer T5 in series with the low pass filter X9. The low pass filters X9 and XI I may be of any type and the inputs thereof are connected by attenuating lnetworks AT and AT' to the outputs of the ra- .dio receiver and microphone.

. The'secondary Winding of transformer T5 is coupled to an inverter stage 22, while the secondary winding of transformer T9 is coupled to an inverter stage 26. These inverting converter stages are of the full wave type, each including a tube system having two diodes and a triode in a common envelope. The secondary winding of the transformer T is Vin two parts separated vby a potentiometer resistance 10, the electrical center of which is connected by a rectifier load impedance I2 to the tube cathode. The free terminals of the secondary winding of transformer T5 are coupled to the diodes of the tube for full wave rectification. When rectication takes place the potential drop through load resistor 'I2 is. applied by capacitor 'I4 to vthe control grid 'I6 so that variations in the potential drop which take place in resistance 'I2 are amplified by the triode section of the tube and fed by the transformer T6 to the low pass filter XI 0. A similar rectifier action takes place in the converter' 26 and channel #2. Since in this converter the operation is the saine as in the converter and rectier 22 the same will not be described. Reference numerals corresponding to those used in the converter 22 plus I0 have been used in the converter 26.

The oscillations developed in are coupled through the load impedances 48 and 50 into the transformers T9 and T5. These oscillations beat with the signals from low pass filters XII and X9 in the mixing and converting stages 26 and 22 and the converted currents are supplied as output through low pass filters XI2 and XID, which select the difference frequency components.

The currents representing signals supplied to X9 and XII may be limited to a range of say 300 to 2700 cycles per second. The low pass filters X9 and XII are then adjusted to cutoff at about 2700 cycles. Filters XIII and XI2 are similarly adjusted. The source 20 may then be of the order of 3000 cycles per second, in which case the diiference output of converters 22 and 26 is selected by lters XII] and XI 2 and in both channels the signals are inverted. That is, in each channel a signal fed to the converter therein at a frequency of say 2700 cycles per second is represented in the converter output by a component having a frequency of 300 cycles, while a signal fed to the converter in said channel at a frequency of say 300 cycles per second is represented in the converter output by a component having a frequency of 2700 cycles per second. Now if the two channels are used in a two-way communication system the inverted signals from channel #2 may be supplied to a transmitter, while the inverted signals in channel #l may represent the output of a receiver of the transmitted inverted signals from a distant station having similar equipment, in which case the converter at 22 reinverts the signals to supply readable si'gnals through transformer T5 and lter XI 0 to the telephones.

In systems of the type illustrated as known heretofore, a bridge circuit arrangement has been used to couple the carrier source 20 to the #l and #2 channels. Then the bridge has four impedance arms with one diagonal in parallel to the primary winding of a transformer in the position of T5 and the other diagonal in parallel to the primary winding of a transformer in the position of T9. The generator 20 in these bridge arrangements would feed a transformer primary winding the secondary of which is in series with the impedance of one bridge arm. These bridge circuits permitted too much coupling between the channels and this resulted in cross-talk. To reduce the coupling and cross-talk attenuation was introduced, thereby lowering the efciency of the system.

The cathode follower stages arranged, in accordance with our invention, between the source 20 and channels #l and #2 reduce the coupling between the channels and thereby reduce the cross-talk and the need of attenuation. In the bridge circuit arrangements, substantially the full carrier voltage was supplied to the filters X9 and XI I, which had to provide a high attenuation .to voltages of the carrier frequency vin .order to prevent them from being fed back through lter XI I to the hybrid coil 9, and thence tothe subscriber station. The low pass filters X9 -Aand XII stop feedback at the carrier frequency to some extent, but not completely. Feedback through lter X9 takes place in like manner, but since it is not connected to theV hybrid coil 9, this feedback is. not 0f importance.

In the improved arrangement of the present invention an increased attenuation of carrier feedback has been obtained by connecting the source of carrier voltage, say impedance 48, the isolating and coupling capacitor 52, the primary winding of transformer T9 and the filter XII (which is shunted by a terminating resistor R6) all in series. The primary winding of transformer T9 has a much higher impedance tothe carrier voltage than either'source 48 or filter XI I and resistance R6 in parallel. Thus only a fraction of the carrier voltage appears across the filter XH terminals, whereas formerly the full carrier voltage was applied to these terminals. Thus for the same filter the voltage fed back through hybrid coil 9 to the subscriber station is reduced by this same fraction (in addition to the attenuation of the filter XH at the carrier frequency). Alternatively, for a permissible amount of voltage fed back, a simpler and more economical filter may be used at Xl l. It is desirable if not essential to choose a type of lter at XH having a characteristic impedance at the carrier frequency which is less than the value of characteristic impedance in the pass band of the iilter, because this aids in keeping the voltage drop at carrier frequency across the filter XH to a low value. However, even if a type of filter having a higher than normal impedance at the carrier frequency is used, R6, which will have a value approximately equal to the normal characteristic impedance, will limit the parallel impedance of it and lter XII to a value less than that of the primary winding of transformer T9.

Carrier feedback through filter X9 is reduced in like manner by the arrangement of capacitor 54, transformer T5, filter X9, etc., because channel #l and its coupling to source 20 are similar to channel #2 and its coupling to source 20.

In mixer systems of this type known heretofore the carrier voltage was applied to the transformer T5 and the filter X9 in parallel. Since the lter X9 is of relatively low impedance, a moderate amount of power had to be supplied by the oscillator 20. This reduced the oscillator stability and increased its distortion. The improved circuit provides for supplying the carrier voltage through the primary winding of T5 and the lter X9 in series. Since the primary winding of T5 is of much higher impedance than the filter X9, the amount of power which must be supplied in the improved system is very much less than formerly. Furthermore, the oscillator 20 now need only supply voltage to the very high impedance of the grid circuit of the coupling stage CS. Therefore, the carrier power required from the oscillator 20 is minute compared with what was required heretofore. In this way We provide an improved method of mixing the low frequency carrier with the audio signal in the same transformer winding, but at a higher impedance to the carrier. The method of mixing in channel #2 is similar and need not be described in detail.

By using like values of circuit components in the couplings to each channel like amounts of carrier voltage are supplied to each of the inver. ter stages 22 and 2B. The inverter stages preferably are balanced and are substantially similar in operation. The gain through the channels, i. e., the audio frequency inputs thereto, is adjusted by adjusting the potentiometer and/or networks AT and AT. To insure proper elimination of the fundamental from the outputs of the inverters 22 and 26 the potentiometers 'E0 and 80 are adjusted to get balanced diode rectification in the inverter triode diode systems. When these adjustments are carried out properly equal outputs may be obtained at the outputs of filters X and X12.

As stated above, the two channels may be put to any use and the filter Xl0 may supply telephones or similar utilizing equipment while the lter XI2 may supply a transmitter or similar utilizing equipment. In the particular application the output of filter XI 0 is supplied, as stated hereinbefore, by way of a hybrid coupling unit 9 to the lines running to a subscribers station, at which a telephone set is operated by the eutput of filter Xl. A microphone may also be located at the subscriber station, to supply currents representing voice signals through the unit 9 to the input of channel #2. The input of channel #l may be the output of a detector in a radio receiver.

What is claimed is:

1. In a modulator, an electron discharge rectiiier system having input electrodes and having output electrodes coupled to an output circuit, a transformer having a secondary winding coupled to the input electrodes of said rectifier system, a cathode follower stage having an input and having a cathode impedance, means coupling said'cathode impedance in series with the primary winding of said transformer, means for impressing modulating potentials across said series coupling, means for impressing oscillations of carrier wave frequency on the input of said cathode follower stage.

2. In a secret signalling system, means for inverting the frequency of alternating currents representing signals comprising, a source of alternating currents representing signals, a transformer having a primary winding and a secondary winding, a source of oscillatory energy of a frequency higher than the highest alternating current frequency, means connecting said source of alternating currents representing signals, said primary winding, and said source of oscillatory energy in series in the order given, a rectifier system coupled with the secondary winding of said transformer, and means coupled to said rectifier system for selecting currents in a band of frequencies covered by the difference between the frequency of said source of oscillatory energy and the frequencies of the currents representing signals.

3. In apparatus for converting alternating currents of one frequency to alternating currents of another frequency, a transformer having a primary winding and a secondary winding, a source of alternating current of said one frequency, a source of heterodyning oscillatory energy, means connecting said sources of alternating current, said primary winding and said source of oscillatory energy in series in the order given, and a rectifier system coupled with the secondary winding of said transformer.

THOMAS E. JACOBI. WILLIAM L. GENSEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,522,044 Bown Jan. 6, 1925 1,571,010 Kendall Jan. 26, 1926 2,051,493 Lunnon et a1 Aug. 18, 1936 2,109,561 Wright Mar. 1, 1938 2,224,690 Moody et al Dec. 10, 1940 2,233,927 Thomas Mar. 4, 1941 2,238,236 Terman Apr. 15, 1941 2,240,420 Schnitzer Apr. 29, 1941 2,240,580 Severin May 6, 1941 2,281,020 Bruck Apr. 28, 1942 2,300,999 Williams Nov. 3, 1942 2,315,567 Webb Apr. 6, 1943 

